TWI Industrial Member Report Summary 231/1984
By R J K Nicholson, J L Robinson and D F Pargeter
Cast-to-cast variations in weld bead shape and penetration can occur during the mechanised autogenous TIG welding [GTA welding] of steels nominally of the same type. This problem has been examined for a range of austenitic stainless Steels (304, 304L, 316, 316L, 347 and 321) and a low alloy steel (2.25Cr1Mo). Mechanised TIG welding equipment and procedures were developed giving reproducible results for a given cast of steel. Autogenous melt runs were made in the experimental steels. Fusion behaviour was assessed primarily with the weld bead depth to width ratio, (DIW). Conduction-limited laser and electron-beam (EB) welds also displayed variable fusion behaviour, weld shape varying in a similar way to that in TIG welds in the same steels. Material characteristics are thus of major importance in penetration behaviour, although additional arc effects are not discounted. Fusion behaviour did not correlate with any aspect of microstructure. Good correlation for one welding condition and material thickness was found with overall chemical composition, for both austenitic stainless and low alloy steels. In the stainless steels, increases in S and P increased penetration, whereas increases in 0 , Si and Mn decreased it. No single elements were identified as of major significance for the 2.25Cr1Mo steels. A computer model was used to predict the influence of several temperature dependent material parameters on fusion behaviour. Isotropic changes in thermal conductivity in either the weld pool or surrounding material were predicted to have little effect. Melting range was also indicated to have small importance. Anisotropy of thermal conductivity, and changes in heat input, did influence pool shape. The results overall support the conclusion that weld pool fluid flow is a major factor controlling penetration. Some authors have suggested liquid metal surface tension to be important in controlling weld pool flow. Surface tension studies did not provide firm evidence for such an influence.